Structural Asymmetry Across Faults of the San Andreas and North Anatolian Fault Systems: Implications for the Structure of Large Faults and for Preferred Direction of Earthquake Ruptures

Ory Dor¹, Yehuda Ben-Zion², and Thomas K. Rockwell^3
1Geological Sciences, Brown University, Providence, RI
²Earth Sciences, University of Southern California, Los Angeles, CA
³Geological Sciences, San Diego State University, San Diego, CA

Theoretical considerations imply that ruptures on large faults that juxtapose different rock types will propagate preferentially in the direction of motion of the block with slower seismic velocity, and that more rock damage will accumulate at shallow depth on the side of the interface that has faster seismic velocity (Ben-Zion and Shi, 2005).

To test this hypothesis, expressions for structural asymmetry across the slip zone of major faults in the southern San Andreas and the North Anatolian fault systems were mapped at various scales at several sites along each fault. Shear fabric on a cm to meter fault-core scale, subsidiary faults and fault rocks on a 10’s of meters fault-zone scale, and pulverized rocks on a 100’s of meters damage-zone scale show systematic damage asymmetry. For the San Andreas, San Jacinto and Punchbowl faults the northeast side is more damaged. Field relations and microfracture pattern in young sediments suggest that the observed damage was generated at a shallow depth. For the 1943 and 1944 rupture zones on the North Anatolian fault, the south and north sides are more damaged, respectively. The asymmetric damage patterns are compatible with preferred rupture directions northwestward on the examined fault sections of the southern San Andreas system, and eastward and westward on the 1943-1944 rupture sections of the North Anatolian fault, respectively (as occurred in these two recent earthquakes). Tomographic studies (e.g. Fuis et al., 2003) show that the more damaged northeast sides of the San Andreas and San Jacinto faults are the blocks with faster seismic velocities at depth.

The results are compatible with the theoretical predictions for bimaterial ruptures, and may have profound implications for earthquake physics, fault zone structure, hazard analysis, and for mining and engineering situations with faults and other material interfaces.

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